feat: add markdown alerts support (#1221)

* Markdown alerts

* refactor(alert): move alert color to variable.scss and simplify the style

* style: fix indent in render-blockquote

* feat(alert): add customizable alert icons for different alert types

* doc: add example usage of markdown alert

---------

Co-authored-by: Jimmy Cai <jimmy@cai.im>

layouts/_markup/render-blockquote.html

@@ -0,0 +1,20 @@
+{{- $type := .AlertType -}}
+{{- $icons := site.Params.article.alertIcon -}}
+{{- $icon := index $icons $type -}}
+
+<blockquote class="{{ if $type }}alert alert-{{ $type }}{{ end }}">
+    {{- if $type -}}
+        <div class="alert-header">
+            <span class="alert-icon">
+                {{- $icon -}}
+            </span>
+            <span class="alert-title">
+                {{- title $type -}}
+            </span>
+        </div>
+    {{- end -}}
+
+    <div class="alert-body">
+        {{ .Text }}
+    </div>
+</blockquote>